1. Field of the Invention
The present invention relates to high temperature coatings of the MCrAlY type and their application to superalloy substrates, particularly by plasma spray techniques.
2. Description of the Prior Art
It is well known that the family of high temperature, oxidation-corrosion resistant coatings commonly referred to as MCrAlY coatings can markedly extend the service life of gas turbine blades, vanes and like components; for example, see U.S. Patents to Evans et al. U.S. Pat. No. 3,676,085; Goward et al. U.S. Pat. No. 3,754,903 and Talboom Jr. et al. U.S. Pat. No. 3,542,530, all of which are of common assignee with the present invention. The MCrAlY coatings are referred to as overlay coatings denoting the fact that they are deposited on the substrate as an alloy and act substantially independently of the substrate in providing oxidation-corrosion protection.
In the past, these coatings have been applied to superalloy substrates by vacuum vapor deposition, sputtering and plasma spraying techniques. Of the three, plasma spraying exhibits greatest versatility in manufacturing operations. However, in plasma spraying MCrAlY coating alloys on superalloy substrates, the prior art has experienced less than satisfactory results due to the development of interconnected as well as isolated pores, voids and like defects in the coating, some of which extend to and penetrate the outer or free surface of the coating. It has been observed that such defects adversely affect the oxidation-corrosion resistance of MCrAlY coatings, for example, as compared to that of similar vapor deposited coatings.